Photoimageable compositions useful as resists for forming printed circuits, printing plates, solder masks or the like have been used for some time now. The earliest photoresists were solvent-borne and solvent-developable. The development of aqueous-developable resists represented an advance in reducing solvent emissions in the immediate workplace and in the general environment. The continued emphasis in reducing organic solvent both from the workplace and from the general environment has spurred the search for waterborne photoresists which are formulated and applied as aqueous liquids. U.S. Pat. No. 5,045,435, the teachings of which are incorporated herein by reference, describes a waterborne photoresist composition which is developable in alkaline aqueous solution. The photoimageable composition described in U.S. Pat. No. 5,045,435 is a primary resist used to form a printed circuit board. After a primary resist has served its purpose in forming the printed circuit board, it is stripped from the board.
Resists are also known which are intended to form a hard, permanent layer on a printed circuit board or the like, for example, a solder mask. A solder mask must be hard, durable, and resistant to chemicals, such as organic solvents and strong acids and bases. In this regard a solder mask should meet the standards of IPC/ANSI SM-840B Table 12 tests, Summary of Criteria for Qualification/Conformance (Institute for Interconnecting Circuits). Herein, a solder mask is defined as a layer which meets those tests. Examples of solder mask-forming compositions that are developable in alkaline aqueous solutions are U.S. Pat. Nos. 5,229,252 and 5,364,736, the teachings of each of which are incorporated herein by reference.
It would be further desirable to have a photoimageable composition useful for forming a permanent layer, such as a solder mask, which is waterborne, thereby eliminating or substantially eliminating organic solvents at the point of application.